I have a Weil Mclain WGO-5 boiler. ITT Bell Gossett HFT-60 diaphragm expansion tank next to the boiler. When the boiler is off the pressure gauge says 3 PSI and temperature around 167 degrees. During a long burn the pressure rises to 25-28psi and the temperature hits 175 area. No water leaks from the pressure release valve during the burn. When the system shuts off some water comes out via the PRV. I am emptying a 5 gallon bucket every 5 days. In November 2018 burner technician replaced PRV and problem abated briefly. Expansion tank taps "hollow" on the bottom 1/3 when system off. Any ideas?